A practical and advantageous construction for a cryogenic food product freezing tunnel is disclosed in Kent U.S. Pat. No. 3,757,533, Sept. 11, 1973. In that freezing tunnel, the tunnel conveyor, circulating fans, cryogen spray header, and cryogen gas exhaust plenum are all mounted on a stationary structure with the cover of the tunnel. The base of the tunnel housing is of U-shaped construction and is mounted on a series of elevators so that the base can be lowered from the tunnel cover to a service position, affording free service access to the tunnel conveyor and other mechanisms.
A freezing tunnel having an elevator-mounted base presents substantial and difficult technical problems if the tunnel is built up from modular units, a type of construction that is usually preferable due to the length of the tunnel, which is often sixty to eighty feet or more. Thus, with an elevator-mounted tunnel housing base comprising two or more modular base units, it is difficult to maintain an adequate seal at the interfaces between the base and the tunnel cover and between the base units if conventional mechanical latching arrangements are employed; a secure high-pressure seal achieved when the tunnel is first closed and is still warm may deteriorate markedly as the tunnel reaches its extremely low operating temperature, down to about -320.degree. F. If the tunnel is provided with complete stationary end sections, as is preferable for mounting of the conveyor drive, exhaust plenum, and other auxiliary equipment, it is also difficult to obtain an adequate seal between the elevator-mounted base unit and the end sections of the tunnel housing. A unitary construction for the tunnel base of the tunnel cover, on the other hand, is also unsatisfactory, particularly for long tunnels, due to the high thermal stresses and the resultant structural distortion.
In any cryogenic freezing tunnel, it is highly desirable to apply the cryogen to the food products or other articles being frozen as a liquid spray; a vapor or partly vaporized spray is substantially less efficient. The cryogen, however, being under substantial pressure at the supply, tends to vaporize in the header and produce an undesirable mixed spray of gas and liquid. This can result in a substantial decrease in the efficiency of tunnel operation.
A related problem is the application of the cryogen to the bottom surfaces of the food products being frozen. The aforementioned U.S. Pat. No. 3,757,533 presents an arrangement for re-use of the liquid cryogen by collecting it in a pan below the spray header and allowing the conveyor belt that transports the food products through the tunnel to sag into the collection pan and thus bring the bottom surfaces of the food products into contact with the liquid cryogen. This arrangement, however, presents a substantial difficulty in maintaining the food products in aligned registration on the conveyor belt. The sag in the belt tends to shift the food products about on the belt surface. Furthermore, if the belt extends completely into the liquified cryogen, the food products may float on the cryogen. Either way, a jumbled and disorganized output from the tunnel results.